Tennis has been one of the most favorite sports in the world. This is due at least partly to improvements in the rackets. Sports equipment manufacturers and tennis players have spent their time improving rackets for decades, such as Taiwanese Patent Nos. 74,410, 76,316, 77,472 and 77,890. These are just a few examples. These disclosures, however, are not related to improvements concerning the cross section of a tennis racket frame. To the best of the applicant's knowledge, there have been no major changes under modifications of the cross section of tennis racket frames heretofore. Conventionally, a racket frame has a substantially rectangular cross section, as shown in FIG. 15, which is a cross-sectional view taken along line 15--15 of FIG. 14 wherein a conventional racket is shown. The cross section is uniform throughout the whole frame (see FIG. 16). The rectangular cross section, however, has the following disadvantages:
1. In accordance with analysis, the major factor that affects the characteristic of a racket is the cross-sectional shape thereof, because it determines, at least partly, the bending rigidity of the racket frame. (The bending rigidity is the area moment of inertia, which is determined by the cross-sectional shape times Young's modulus of the material that the frame is made of.) Further, there are two major external loads that will act upon the frame. One is the impact force resulting from hitting a tennis ball; this force is substantially perpendicular to the plane of the racket head. The other major load is the tensile of the strings and/or impact force resulting from a sudden contact of the frame tip with the ground during play; these forces are generally parallel with the plane of the racket head. Accordingly, the frame should have a structure that is capable of supporting parallel or lateral loads near the frame tip, and the structure close to the percussion region should be more rigid in the direction of hitting a ball. With a uniform frame as in a conventional racket, it is not possible to react to both kinds of external loads efficiently and effectively.
2. The drag coefficient of air for a rectangular cross section, such as in a conventional racket, is large, thus resulting in a great resistance against the movement of the racket.
3. The bending rigidity of a rectangular cross section is less than an elliptical one with the same material and same cross-sectional area, in accordance with the theory of the strength of material. The lower the bending rigidity, the larger the deflection. A large deflection results will apparently in less precise control of a ball's movement.
Accordingly, a racket with a uniform rectangular cross section is less efficient and effective in playing. Besides, a uniform rectangular cross section also results in greater stress in certain locations when acted upon by external loads.